Nanometric metrology is the foundation of many applications such as nanofabrication, nanomanipulation and nanomaterials. IC, precision machinery, micro-electromechanical system (MEMS) and many other industries all require high-resolution, high-precision displacement sensors for nanopositioning. With the fast development of ICs toward large-scale and high-integration ones, the requirements on overlay accuracy of photolithography tools and hence on accuracy of six-degree-of-freedom (6-DOF) position information of wafer and mask stages are becoming more and more demanding.
Interferometers have been employed in photolithography systems to measure positions of wafer and mask stages due to their high measurement precision of nanoscale. However, existing interferometers are almost at their precision limit and suffer from susceptibility of their precision to surroundings, low measurement repeatability and errors typically exceeding 1 nm even in good ambient conditions. Therefore, it is difficult for traditional interferometer-based measurement system to meet the still-increasing overlay precision demands, and there is an urgent need for high-precision solutions allowing measurements on the order of picometers with high consistency.
Grating-based measurement systems operate with good repeatability while not being affected by surroundings much. They are increasingly replacing interferometers in the most recent photolithography systems to perform precise, stable measurements on the order of picometers. U.S. Published Patent Application No. U.S. Pat. No. 7,389,595 proposes a two-dimensional grating-based measurement system based on fiber optic transmission of light from a light source and that carrying detection signals. In this solution, the light source is a semiconductor laser, and a displacement of the grating relative to a read head is measured in a homodyne manner. However, such homodyne detection is vulnerable to interference, and the collected position data are susceptible to external stray light, electromagnetic fields and vibration. Chinese Patent Application No. CN201210449244.9 proposes a dual-band heterodyne grating-based measurement system providing effectively increased measurement precision. However, only detection signals are transferred by fiber transmission, with a laser light source and a grating reading head being integrated to lead to a bulky structure, making it unsuitable for use in tight space applications. Moreover, when there is any Rx/Ry angular offset between the grating and the read head, the interferometric performance of the measurement system will be degraded, leading to failure of the system. Further, due to very high complexity in assembling and commissioning of the grating and the reading head, this system does not allows ease of installation and use.
U.S. Published Patent Application No. U.S. Pat. No. 8,300,233B2 proposes a grating-based measurement system in which a light beam is perpendicularly incident on a grating and a corner reflector reflects the resulting diffracted beams back for determination of two-dimensional position data in horizontal and vertical directions. Limited to the structure of the corner reflector, any vertical relative displacement between the grating and a probe will cause a deviation between reference and measuring spots formed on a receiver, and in case of complete separation between the reference and measuring spots, optical interference will be impossible. Therefore, limited by the spot size, this measurement system suffers from a small vertical measurement range between the grating and the probe.